The present invention relates generally to fluorescent lamp control devices, and particularly to a dimmer control function in a fluorescent lamp used in a liquid crystal display (LCD) device of an aircraft instrument display system.
LCD panels are now used widely in many aircraft instrument display systems. An LCD device includes a liquid crystal panel selectively made opaque in certain regions in order to generate images, icons and characters in an instrument display in response to, for example, a video signal. To further enhance the visibility of such images of the liquid crystal panel, LCD devices require a backlight, i.e., a light source positioned on the backside of the liquid crystal panel. As applied to aircraft instrument display systems, especially in military aircraft display systems, it is important that the LCD device maintain a substantially constant display luminance. As may be appreciated, the pilot of such an aircraft is better able to observe LCD presentations with constant luminance. Variation in LCD luminance can be distracting to a pilot operating the aircraft, especially a fighter aircraft engaged in combat maneuvers. Accordingly, it will be understood that the maintenance of constant luminance in an aircraft display system is not only a desirable characteristic, it can be vitally important when the pilot makes split-second decisions based on information obtained from the LCD instrument display system.
Various factors can affect both the perceived and actual luminance of an LCD instrument display. For example, temperature variations can affect the light output of a fluorescent lamp, and, therefore, the actual luminance of an LCD device using a fluorescent lamp as the backlight device. Variations in ambient light conditions affect the perceived luminance of an LCD device. An aircraft instrument display system should provide substantially constant perceived luminance through a range of bright daylight to extreme darkness. Bright daylight conditions require a relatively greater actual light output to maintain a given apparent luminance of the LCD display device. For extreme darkness, a relatively lesser light output is required to maintain constant the perceived luminance of the LCD device. For temperature variations, extreme high or low temperature conditions, i.e., relative to a most efficient temperature condition for a given lamp, require greater energy input to the fluorescent lamp in order to maintain a given LCD luminance. As may be appreciated, the requirements for aircraft, especially military aircraft, are stringent. The temperature and ambient light conditions through which constant perceived and actual luminance are required are broad.
Previous aircraft display systems directed toward relatively constant LCD display device luminance have used dedicated circuitry in the control of light energy output from the fluorescent lamp of the LCD device. For example, various potentiometers and dedicated analog circuitry have been used in conjunction with frequency generators in order to provide dimming functions of fluorescent lamps. Voltage divide circuits have been used to establish temperature set points in the operation of the dimmer circuits as a function of ambient temperature.
The pilot typically controls the brightness of an instrument display by adjusting a potentiometer either on the particular display itself or somewhere on the cockpit instrument panel. Since the eye of the pilot perceives luminance logarithmically in response to linear brightness changes, elaborate analog circuitry has been used to make the perceived logarithmic change in display brightness more uniform in relation to linear potentiometer rotation. Thus, some transformation function is required between the system input provided by the pilot, e.g., operation of a potentiometer, and the operation of the LCD device. In prior fluorescent lamp dimming circuits this transformation function was inflexible as embodied in dedicated circuitry. If, for example, a change in this transformation function was desired, e.g., by preference of a given aircraft purchaser or particular unexpected system configuration, significant design and manufacture changes in the dedicated dimming control circuitry were required.
According to another aspect of fluorescent lamp dimming circuitry, it is important that the dimming circuit not over-drive the fluorescent lamp and thereby deteriorate the lamp. Aircraft display instruments must be as reliable as is possible. Each lamp is desirably operated in an optimum fashion which provides a required display luminance while not over-driving, and therefore deteriorating, the fluorescent lamp. In prior dedicated fluorescent lamp dimming circuits, it has been difficult to design a simple dedicated fluorescent lamp dimming circuit which delivers the required display luminance while not over-driving the fluorescent lamp. Significant complexity in such dedicated dimming circuitry is required to achieve these design goals. Accordingly, prior fluorescent lamp dimming circuits have necessarily traded display luminance control for the reliability, i.e., life expectancy, of the fluorescent lamp.
The present invention provides a fluorescent lamp dimming control function addressing these shortcomings of the prior dimming systems and is well suited for use in aircraft display systems for improved overall pilot operation.